Authors' reply to: "The role of long-term effects of allopurinol on cardiovascular outcomes and all-cause mortality in diabetes"Our study found an association between allopurinol and reduced risk of mortality and cardiovascular outcomes in people with diabetes; however, although this association was observed in analyses that classified time as exposed or unexposed to allopurinol and in analyses that classified time according to current dose categories, we found no association in analyses classifying time according to cumulative allopurinol dose.Dr Abtahi raises methodological considerations regarding how we evaluated allopurinol exposure (specifically the possibility of allopurinol exposure misclassification, and not evaluating for a chronic effect of allopurinol on the outcomes of interest).We agree there are strengths and limitations of our study design, as with any study design. We strongly felt that any comparison of allopurinol-treated and untreated groups would be susceptible to serious residual confounding because important confounders related to the selection of treatment are not available in administrative data and thus cannot be accounted for, even when using advanced methods of adjustment such as propensity scores.We do not agree with Dr Abtahi's assertion that our analysis was susceptible to exposure misclassification, which is incorrect attribution of exposed follow-up time to unexposed follow-up time, or vice versa.Rather, Dr Abtahi's concern regards a biological lag, in which the effects of allopurinol might not manifest for a period of time after initiation, or would endure for a period of time following discontinuation. 1 We selected a lag time/grace period of 14 days to address the potential of reverse causality (ie, discontinuation of allopurinol as a marker of impending outcomes). Furthermore, if a biological lag were present and considered in the analysis, then time following allopurinol discontinuation that is low-risk for events would be classified as exposed time. This would result in an even lower hazard ratio than we observed. Thus, our results, as presented, were a conservative estimate if a biological lag exists.Our analyses evaluated both acute and chronic benefits of allopurinol.The time-varying binary classification (exposed vs. unexposed) and timevarying current dose analyses evaluated allopurinol's acute effects, while the time-varying cumulative dose analysis evaluated its chronic effects. If long-term (chronic) allopurinol use was associated with mortality and cardiovascular outcomes, we would have expected to observe an association between cumulative allopurinol doses and the outcomes, which we did not. One possible mechanism by which allopurinol may have protective benefits is through reduction in oxidative stress, and it is reasonable to surmise that this may be an acute effect. Indeed, two randomized trials of allopurinol administered for 4 weeks and 8 weeks demonstrated improvement in endothelial function and left ventricular function over these short time periods. 2,3 Thus...